This invention relates to a microflow sensor element for detecting a flow of gas and a manufacturing method thereof.
In a conventional microflow sensor chip of the sort shown in FIGS. 9A and 9B, a heater portion b1 disposed in the center of a sensor chip a is heated by an electric circuit (not shown) to a temperature higher by a predetermined degree than the temperature of a fluid to be detected and when no air flow exists, the temperatures of temperature sensors c1 and d1 symmetrically arranged on both sides of the heater portion b1 are equal. Circular arcs of broken lines indicate isotherms.
When the air current is received, however, there is produced a temperature difference between both the temperature sensors c1 and d1 because the symmetry of the temperature distribution is broken, and the temperature of the temperature sensor c1 on the upwind side becomes lower than that of the temperature sensor d1 on the leeward side. The temperature difference then causes the resistance value to vary, which is converted into an electrical signal whereby to obtain an detection output.
Since the detection output is obtained from a change in the temperature distribution in the case of the aforementioned microflow sensor element, the drawback is that the detection output varies according to the temperature difference in the direction of the air flow. Consequently, it has been arranged that an air flow in a substantially laminar-flow condition is passed over the element in a fixed direction.
When the air flow is detected in a closed space, for example, the direction of the air flow tends to vary and besides air turbulence may occur, which makes it difficult to gain highly accurate measured values from the aforementioned conventional microflow sensor element.
Another thermal flow detector element is shown in FIGS. 10 and 11, which consists of two heaters a2 that are made of a metal (e.g. Ni) foil and that run in a serpentine path and plates b2 that are made of an insulating material such as glass and which support the two heaters a2 in a face-to-face relationship. The plates b2 have an opening c2 and the gaps d2 between adjacent branches of each heater a2 that are located within the opening c2 provide gas flow channels.
With a constant voltage being applied to the heaters a2 so that their temperature becomes higher than that of the gas in the gas compartments by a certain value, the two heaters a provide the temperature profile shown by (i) in FIG. 11 if there is no gas flow. However, if the gas flows through the gaps d2 as indicated by an arrow, the heater a2 in the upstream position is cooled in accordance with the gas flow rate whereas the heater a2 in the downstream position is heated with the heat taken from the upstream heater a2; as the result, the two heaters a2 provide the temperature profile shown by (j) in FIG. 11. This temperature change causes a change in the resistance of the heaters a2, which is measured with a Wheatstone bridge or the like, thereby detecting the gas flow.
The conventional thermal flow detector element shown in FIGS. 10 and 11 has the following problems. Since all gaps d2 between adjacent branches of each heater a2 that are located within the opening c2 serve as gas flow channels, the channel or flow path area is large compared to the area of the detector element and the gas flow rate is so much retarded that the change in the temperature of the heaters a2 due to the gas flow is insufficient to provide high sensitivity.
That part of the heaters a which is located within the opening c2 (i.e., which is not supported with the plates b2) must have a sufficient strength to retain shape, so a thick enough metal foil has to be used to compose the heaters a2. However, if the thickness of the heaters a2 is increased, the heat capacity increases correspondingly to slow down the response speed.
In view of the actual situation mentioned above, an object of the present invention is to provide a microflow sensor element for making obtainable a response quickly with high response sensitivity regardless of the direction of an air current.
According to the present invention, a pyroelectric detection unit is provided on a substrate, the pyroelectric detection unit having an upper and a lower electrode which are respectively provided on the surface and undersurface of a ferroelectric thin film, and that a heater portion whose temperature is modulated periodically as designated is provided on the upper electrode via a thin film of insulating material.
Further, according to the present invention, through-holes for use in gas venting are provided in or around the pyroelectric detection unit.
By letting an electric current flow periodically into the heater portion, a pyroelectric current can be detected in the pyroelectric detection unit in proportion to the degree of a change in the temperature rising then. The detection output is decreased because the temperature of the heater portion is restrained from rising by a flow of gas while the flow of gas is existing around the microflow sensor element. Thus, the flow rate of the gas is made detectable by the output difference.
Since the pyroelectric detection unit is provided just below the heater portion, it is possible to detect any gas movement quickly with high response sensitivity, however little the gas movement may be and regardless of the direction of the gas movement.
Moreover, the provision of the through-holes for use in gas venting in or around the pyroelectric detection unit allows the detection of even the flow of gas in the direction of a perpendicular plane with respect to the pyroelectric detection unit.
Furthermore, according to the present invention, a gas passage hole whose setting of channel area is smaller than the area of the gap between adjacent segments of a heater to be supplied with a periodic voltage so that its temperature is a certain value higher than the temperature of the gas in the gas compartments is formed in the neighborhood of said heater.
Since gas passage holes of which the channel area is smaller than the area of the gap between adjacent segments of the heater to be supplied with a periodic voltage are provided near the heater, the flow rate of the gas flowing through the gas passage holes is sufficiently increased that the temperature of the heater experiences a great enough change to provide a higher sensitivity.
Moreover, according to the present invention, a process for producing a flow detector element comprises the steps of forming a lower electrode on a substrate, forming a thin ferroelectric film on the lower electrode, forming an upper electrode on the thin ferroelectric film, patterning the upper electrode, the thin ferroelectric film and the lower electrode in that order, with a gas passage through-hole being also formed, thereafter forming a thin insulator film that covers a pyroelectric sensing portion that is composed of the upper electrode, the thin ferroelectric film and the lower electrode, with a through-hole and contact holes being also formed in said thin insulator film, forming a heating electrode film on the thin insulator film, allowing a portion of the heating electrode film to drop in the contact holes to make lead-ins for the upper and lower electrodes, and removing that part of the substrate which is just under the pyroelectric sensing portion to form an opening, thereby producing a pyroelectric flow detector element.
According to the processes described above, gas passage holes can be formed without regard to the conductor size and pattern of the heater and by reducing the channel area of the gas passage holes, the gas flow rate can be sufficiently increased to provide higher sensitivity. In addition, the heater is supported by the thin insulator film, so there is no need to use a thick heating electrode film and pyroelectric flow detector element can be formed as thin enough films to reduce the heat capacity and thereby increase the response speed. A particular advantage results from the pyroelectric flow detector element which theoretically can produce by far greater signal outputs than the thermal type to achieve a marked improvement in detection sensitivity.